Although recognized as a chronic relapsing disorder, addiction is still largely treated as an acute disorder. To better treat addiction as a chronic disorder, self-management and social support should be provided. Social media provides a unique and cost-effective opportunity to arrange and develop the social support system necessary for sustained recovery. Unfortunately, little is known about the specific methods to develop an efficacious social media outlet that facilitates user self-management, the adoption of health behavior, engagement in efficacious treatment, and the flow of ideas among those in recovery. Advances in social network science have demonstrated that social interactions are complex dynamical systems with emergent properties (e.g., engagement), not reducible to the individuals in those social networks. This proposed project is a systematic effort to translate basic research on the emergent properties of online social networks to support continued addiction recovery. Here we propose to recruit 1536 poly-substance dependent individuals in recovery from the International Quit & Recovery Registry to participate in two intent-to-treat randomized controlled trials. In these trials, we will examine whether topology of the social network (Experiment 1) and the similarity of individuals to one another (Experiment 2) strongly influence social network engagement, exploration, social learning, and involvement in an efficacious treatment. We will also learn whether such engagement supports continued recovery from addiction. Our hypotheses are that topologies with greater redundancy in connections and networks with increased similarity across individuals will increase involvement and engagement among network members, and this increased involvement will translate into decreases in the relapse rate of group members. As drug-related behaviors are strongly influenced by group pressure and social learning, the potential effects of these two variables on engagement with efficacious treatment and others supportive of recovery could have a dramatic impact on the societal cost incurred by individuals who attempt and fail to reach full recovery status.